The invention is in the field of broadband light sources used to calibrate spectrometers or supply light to other optical instruments for powering them or for other calibration and measurement purposes. Spectrometers are instruments which are used to determine spectral information about objects which indicate to scientists and engineers qualitative and quantitative information about the objects. Light emitting objects (which may include planets and samples from planets) emit light in the form of photons when excited by an energy source. Light in the form of photons is emitted when electrons of an object change states in distinct energy levels according to quantum theory. It is this emission which scientists and engineers sense with a spectrometer to evaluate and correlate measurements of the spectrometer with known spectral information about elements to infer the constituency of the object. The light emitting source or object may be a planet or it may be a sample taken from a planet and analyzed in a laboratory type environment on-board a space ship.
Spectrometers have been known for some time but those in use today are highly specialized and designed to process and measure electromagnetic radiation over a certain band of frequencies. Crude spectrometers can be made at home from cereal boxes with slits in them which admit light in a certain region of the visible spectrum to a compact disk placed at an angle with respect to the incoming light which is then refracted in spectra which can be viewed and used to infer the chemical make-up of the atmosphere. Spectrometers used on board spaceships are very sophisticated instruments and include complex calibratable gratings and electronics to accept and process light from planets and samples from planets in determining the elemental composition of an object. Spectrometers analyze the excitation of objects which have been excited by a light source.
Military use of spectrometers is prevalent to identify discontinuities in the surface of our own planet to infer that which is not normal for a specific location thus identifying weapons and equipment.
Electromagnetic radiation is denoted as such because it includes electric fields and magnetic fields which are propagated by the source of the radiation. The general equation of frequency times wavelength=speed of light governs all electromagnetic radiation such that the higher the frequency the shorter the wavelength and vice versa.
The spectrometers which can be used with the broadband light source of the instant invention are designed to view the entire visible light spectrum.
White light sources are used to calibrate spectrometers in the visible range. It is desirable, therefore, to have an output source which spans past the visible light spectrum. The visible light range includes red, orange, yellow, green, blue, indigo and violet. Some recent commentaries are now indicating that indigo should possibly not be included as a separate color as it is a combination of other colors.
A broadband white light source includes all components of the visible spectrum as demonstrated by early scientists such as Newton wherein white light coming from the sun was broken down into its several components with the use of a prism.
U.S. Pat. No. 6,796,866 which has one inventor in common with the instant application is incorporated herein by reference. FIG. 1 is a schematic 100 of the prior art device illustrated in U.S. Pat. No. 6,796,866. There are three silicon layers 108, 106, 102 disclosed in the '866 patent. The silicon substrate 202 includes a top nitride layer 204, a bottom nitride layer 206, and a cavity 208 which allows for the transmission of light to the window 104. The silicon substrate 106 is a middle filament mount layer having a top nitride layer 214 and a bottom nitride layer 216. The silicon substrate layer 108 includes a reflective top layer 242. The spiral filament 220 is bridged across an aperture 218 in the middle substrate 106. Contacts 220 of the filament communicate with electrical leads 222 which in turn communicate with wire bond pad 210 and wire bond lead 240.
The '866 patent discloses a MEMS based package which employs Ti/Pt/Au bonding rings 230, 232, 250, 252 to bond the bottom layer 108 to the middle layer 106 and the middle layer 106 to the top layer 102. The bonding rings are deposited on the Si semiconductors in a facing relationship. Insulation 234 is interposed between the nitride 214 and the contact pads 232 to prevent electrical shorting between electrical leads 222 and conductive bonding rings 232.
The bottom layer 108, the middle layer 106 (silicon substrate 212), and the top layer 102 are bonded together. The reflective surface 242 contributes to the radiation of light through transmission window 104 in the top nitride layer 204 of the top layer 102.
FIG. 2 is a schematic 200 of the prior art device illustrated in U.S. Pat. No. 6,796,866. The end contact portions 220 are connected to electrical leads 222, 236 according to the specification of the '866 patent. It will be noticed that the spacing between the double-spirals is constant between the end portions 220 and an intermediate portion 291 all the way to the central portion 292. The language of a double-spiral, intermediate portion 291 and the central portion 292 does not appear in the '866 patent and are not extracted therefrom. It has been discovered that when the filament of the '866 patent is heated from the joule heating caused by the flow of current therethrough, the filament 200 expands radially and it also expands lengthwise.
Actual mechanical and electrical contact may result in partial shorting of a portion of a turn of the spiral. If the turns are too close together this can happen due to some distortion of the spiral during operation or due to vibration. In an earlier design in which the space between turns was everywhere constant, the shorting tended to occur near the outer part of the spiral where distortion appeared to be most severe.
The filament has constant thickness. The width of the filament varies along its length as wider tabs (end contacts) and less wide outer windings and still less wide inner windings. The thermal expansion of the material differs based on joule heating which varies with the voltage applied and with the resistance (determined by cross-sectional area) of the particular part of the filament. The end contacts of the filament are wider than the windings of the filament so the end contacts or tabs have a larger cross-sectional area than the windings and so have a lower resistance than the windings and so have less joule heating for a particular operating voltage than the windings. Since both the end contacts and outer windings are wider than the inner windings, both the end contacts and outer windings run cooler than the inner windings for a particular operating voltage. The cooler metal expands less and is less flexible than the warmer inner windings. The inner windings expand along their length, following the curve out. As the inner windings tend to de-coil or unwind, their length increases and they may eventually collide with the more stationary cooler outer windings producing an electrical short path.
FIG. 2A is a schematic 200A of the prior art device illustrated in U.S. Pat. No. 6,796,866 with the tungsten coil thereof illustrated in the process of unwinding. Reference numerals 260A, 261A, 262A and 263A indicate the general location where the first and second spirals interengage each other upon heating thereof. In FIG. 2, reference numerals 260, 261, 262, and 263 indicate gaps between the windings of the spirals. When the first and second spirals are heated they expand and engage as indicated by reference numerals 260A, 261A, 262A and 263A as illustrated in FIG. 2A.
The shortened path then has lower resistance and draws even more current which increases the evaporation rate of the filament which decreases the lifetime because the filament narrows further until it gets to a point where it melts and opens the circuit creating an open circuit path.
Although the light source of the '866 patent is a very efficient broadband light source its assembly is somewhat complicated and involves the deposition of the Ti/Pt/Au bonding rings on the top, middle and bottom layers and subsequent processing under vacuum or in an inert atmosphere at or near atmospheric pressure in a thermal compression binder to assemble the device together.
U.S. Pat. No. 5,977,707 to Koenig discloses a planar spiral made from tungsten as illustrated in FIG. 7 thereof but the remaining claimed structure and processes are not found or suggested by the reference.
U.S. Pat. No. 3,604,971 to Tracy discloses a filament mounting structure for a display device which includes a plurality of helical filaments to form a display but fails to disclose a single planar double spiral as claimed.
Therefore, it is desirable to have an ultraminiature light source which does not short-out due to thermal expansion and vibrations of the filament and which is efficiently packaged.